It is common for the femoral component of a cement hip prothesis to require revision after a number of years in use. The revision involves removal of the femoral stem portion of the endoprosthesis anchored in the intramedullary canal. Removal of the endoprosthesis does not itself require a great deal of surgical time The bone cement left, however, does require a great deal of surgical time to remove.
Numerous impact-type chisels and high and low speed drills have been used in various combinations for removing bone cement. A problem facing the surgeon is the lack of control over the impact chisel or drill, since they can drift from origin during use. As a result, the device will occasionally penetrate the femur wall.
Bone cement generally has a hard smooth surface which makes it difficult, if not impossible, to get a good bite with a chisel or drill. This, coupled with the lengthy surgical time, leaves room for improvement.
Bone cement is widely accepted as the leading anchoring medium for the femoral component to the medullary canal wall. In addition, it has been found with great success that once bone cement has been removed, the cancellous bone will grow into a cementless femur stem.
It is imperative that the bone be protected against unnecessary exposure to extreme heat. When bone cement is used to anchor the prothesis in a bone cavity, the bone cement transmits a considerable amount of heat to the bone during its exothermic curing process. These temperatures can approach 80.degree. C. (180.degree. F.) over a period of about four minutes. Normally, this does not damage the bone.
It has been proposed to remove bone cement from the femoral medullary canal during a hip revision using a thermal chisel. Such techniques are described, for example, in U.S. Pat. No. 4,702,236 to Tarabichy, et al. These techniques comprise use of instruments with heated working ends of various shapes heated to a temperature exceeding the melting point of the bone cement. According to Tarabichy, et al., the heated instruments are used to scoop out the cement by penetrating the instrument into the cement by melting the cement, until penetration is arrested by the wall of the bone. Substantially all of the cement is removed from the bone cavity by continued use of these heated instruments until sufficient bone cement is removed for reanchoring the removed prosthesis.
Tarabichy, et al., disclose that the thermal chisel is heated to a temperature above the melting point of the cement, i.e., about 150.degree. to 200.degree. C., in order to soften the cement to the point where it can be removed by the heated instrument.
However, it has been found that the tip of such a heated instrument should operate at a temperature in the range of about 400.degree. C. to about 450.degree. C., in order to plasticize the bone cement, in a practical length of time, for reasonably fast and easy removal. These temperatures are far in excess of temperatures that, by direct contact with the bone wall, can permanently damage the bone and its adjacent tissue.
Certain concerns have been raised as to the thermal effect of these prolonged temperatures on survival of the bone, particularly if substantial contact occurs between the heated working end of the instrument and the bone wall.
The amount of heat transfer to the bone wall is directly proportional to the area, temperature and time of contact of the heat source, i.e., the surface area of the heated tip of the instrument which may contact the bone wall. Since the scoop-type heated instruments disclosed in the Tarabichy et al patent may have a rather large surface area, a potentially damaging amount of heat can be transferred to the bone wall when removing the bone cement. This problem is particularly critical if contact with the bone wall is used as the means for arresting penetration of the heated tip of the instrument, and if the instrument is heated to the higher temperatures which are more effective in removing the cement.
Excessive heat is far more damaging to the bone and adjacent tissue then accidental penetration of the bone with impact chisels of the type used in the prior art for removal of bone cement. Although the bone cement itself provides some protection for the bone during removal of the bone cement, the large area of heat contact between heated instruments and the bone wall can transmit damaging heat to the bone, not only through contact with the heated working end of the instrument, but also with the shaft of the instrument.
Additionally, concerns have been raised about the use of instruments, within the human body, operating at the conventional deadly 120 volts.
The present invention provides a method and apparatus for removing bone cement from a bone cavity by avoiding damaging heat transfer to the wall of the bone when the bone cement is being removed while utilizing voltages of less than 24 volts isolated from the normal utility 120 volt outlet source.